1. Field of the Invention
This invention pertains to the general field of optical communications and, in particular, to a step-phase interferometric optical interleaver.
2. Description of the Prior Art
Interferometric optical interleavers are commonly utilized in dense wavelength division multiplexing (DWDM) optical communication, where various frequencies (1/λ) of laser light are used as carrier signals (channels) and are coupled into the same optical fiber, which acts as a waveguide. Data signals are superimposed over the carrier signals and are transported in the waveguide. Thus, the information capacity is directly proportional to the number of channels in the fiber. Since the total usable wavelength range is limited (about a few tens of nanometers), as channel spacing decreases, more channels can fit into the same optical fiber and greater communication capacity is achieved.
Channel spacing is limited by the capability of the multiplexer (MUX) and the de-multiplexer (de-MUX). Currently, the standard channel spacing is 100 GHz (0.8 nm) and manufacturing costs increase dramatically to implement a channel spacing smaller than 100 GHz. Therefore, a cost-effective method for interleaving channels and enabling the use of higher bandwidth filters with lower channel spacing in an optical communication system would be very desirable. For example, a one-stage interleaver can be used with 100-GHz filters and a 50-GHz channel spacing. Similarly, a two-stage interleaver enables the use of 100-GHz filters in a 25-GHz channel-spacing communication system.
Interferometric interleaver devices used to separate communications channels into distinct optical signals are well known in the art. See, for example, U.S. Pat. Nos. 6,169,626, 6,268,951 and U.S. Pat. No. 6,275,322. These interleavers combine a beamsplitter with an etalon cavity (also known as a Gires-Tournois resonator or GTR) in a non-linear phase-shifter arm (the GTR arm) of the interleaver and a mirror in a reference arm (the mirror arm) to produce the necessary phase shift and interference effects. FIG. 1 illustrates schematically such a conventional step-phase interferometric interleaver. A multi-channel optical input W0 is passed through a beam splitter 10 which splits the beam into a first wavefront W1 transmitted toward a GTR device Mc and a second wavefront W2 reflected toward a mirror M2. The GTR includes a front surface 12 and a parallel back surface 14 with very low and very high reflectance, respectively (which define a cavity also known in the art as an “etalon”). The GTR Mc and the mirror M2 are positioned at distances L1 and L2, respectively, from the interface 16 of the splitter 10. The GTR causes a phase shift in the wavefront W1 which is returned to and partially reflected by the beamsplitter 10 to produce a beam ETCR. in a first output arm of the step-phase interferometer and is partially transmitted to produce a beam ETCT, in a second output arm of the interleaver. Similarly, the wavefront W2 is reflected from the mirror M2, is partially transmitted by the beamsplitter 10 to produce a beam ERMT in the first output arm, and is partially reflected to produce a beam ERMR in the second output arm of the interleaver. The notation used in this disclosure, wherein T, R, M and C refer to transmission, reflection, mirror and cavity (resonator), respectively, and the prime symbol (′) refers to the internal optical path, is conventional in the art.
Several complicating factors limit the degree to which these interferometric interleavers can be utilized to increase communication capacity. For instance, the non-linear dependence of the optical phase on carrier frequency of the interleaver intrinsically introduces undesirable chromatic dispersion that reduces their usefulness. A solution to this problem is disclosed in commonly owned U.S. Ser. No. 10/267,216, hereby incorporated by reference, wherein a GTR is used to cancel chromatic dispersion in the passband of the device.
The proper functioning of optical interleavers is predicated upon the precise control of the optical-path lengths of the wavefronts traversing the device (L1 and L2 in FIG. 1), which are susceptible to variation as a result of manufacturing tolerances and thermal effects. U.S. Ser. No. 10/287,340, also incorporated by reference, provides a solution to this problem based on the use of a parallel glass plate mounted on a rotating fixture in the cavity of the interferometric interleaver. The glass plate refracts the incident optical signal to increase the length of its optical path as a function of the angle of incidence. Thus, the device can easily achieve a precision in the order of nanometers.
Another problem arises from the way interleavers are in fact manufactured in order to simplify the challenge of controlling precisely the optical-path length of each arm of the device. Rather than using a mirror as illustrated in FIG. 1 or a totally reflective surface on a solid block, each of which provides no flexibility in establishing the length of the optical path L2, a cavity with spacers is used. As illustrated in FIG. 2, the structure of such a conventional interleaver device 20 consists of two air spaces between a transparent body incorporating the beamsplitter interface 16 and the reflective mirrors M2 and 14 (see FIG. 1 also). These air spaces are varied to set the cavity length Lc and the distance of the mirror M2 from the beamsplitter interface 16 such that the mirror's distance L, from the interface is equal to the distance from the interface to the middle of the GTR's cavity; that is, L2=L1−1/2Lc. Accordingly, a first set of spacers 22 determines the distance between the two etalon surfaces 12 and 14. A second set of spacers 24 establishes the distance L2 between the beam splitter interface 16 and the mirror M2, thereby also setting the optical-path difference L2-L1. Thus, in practice the mirror arm of the interleaver is also implemented with a cavity structure and the reflectivity of the front surface 26 is critical to the proper functioning of the interleaver. Ideally, the surface 26 should be totally transmissive, which would produce the same effect as the mirror of FIG. 1. In practice, a reflectivity of less than 10 percent is tolerated because of limits in the effectiveness of antireflective coatings, but a corresponding phase error is introduced that severely limits the use of interleavers to improve the capacity of optical communication systems.
As those skilled in the art would readily recognize, in the pass band of optical frequency processed by an interferometric interleaver, the two beams are in phase, causing constructive interference. In the stop band of the optical frequency, the two beams are out of phase and, therefore, interfere destructively. Channel isolation depends on how well destructive interference is achieved. If the interfering beams are exactly out of phase within the stop band (i.e., with a phase difference of 180 degrees), the resulting isolation is perfect. However, any variation from 180-degree in the phase difference between the two interfering beams (herein defined as “phase error”) significantly degrades the performance of the interleaver. The dependence of channel isolation on phase error is given by the relation
                              Channel          ⁢                                          ⁢          Isolation                =                  10          *                                    log              10                        ⁡                          [                                                sin                  2                                ⁡                                  (                                                            ϕ                      error                                        2                                    )                                            ]                                                          (        1        )            where φerror is phase error in radians; and Channel Isolation is defined as the ratio of the residual power to the input power in the stop band, measured in dB. This relation is illustrated graphically in FIG. 3.
In current communication systems, the typical acceptable channel isolation is about 27 dB. Accordingly, the graph of FIG. 3 shows that a maximum phase error of 5 degrees is acceptable, which is very difficult to achieve with traditional interleaver construction. It is well known that a GTR or any etalon cavity structure introduces a nonlinear phase. For an interleaver of the type shown in FIG. 1 (see also B. Dingel et al., “Multifunction optical filter with a Michelson-Gires-Tournois interferometer for wavelength-division-multiplexed network system applications,” Optics Letters, Vol. 23, No. 14, 1998), the phase difference between the beams reflected by the GTR and the Mirror M2 is nearly 180 degrees in the stop band and, therefore, the two beams interfere destructively. The phase difference in the pass band is nearly in phase; thus, they interfere constructively. Any deviation from 180 degrees produces a residual power in the stop band, as illustrated by the intensity plots of FIG. 3A. The local peaks in the stop band determine the channel isolation of the device. The phase error corresponding to the local peak is a local extreme and is given by the relation
                                                                                          ϕ                  NP                                =                                                      2                    ⁢                                                                  tan                                                  -                          1                                                                    ⁡                                              [                                                                                                            α                              ⁡                                                              (                                                                  1                                  -                                                                      2                                    ⁢                                    α                                                                                                  )                                                                                                                    (                                                              2                                -                                α                                                            )                                                                                                      ]                                                                              -                                                            tan                                              -                        1                                                              ⁡                                          [                                                                                                    (                                                          1                              -                                                              2                                ⁢                                α                                                                                      )                                                                                α                            ⁡                                                          (                                                              2                                -                                α                                                            )                                                                                                                          ]                                                                                                                                                                α                  =                                                            1                      -                                                                        R                          NP                                                                                                            1                      +                                                                        R                          NP                                                                                                                    ,                                                    ,        where                            (        2        )            and ′RNP is the reflectivity of the front surface 12 of the cavity in the GTR. Usually, to obtain wide enough pass-bandwidth, the reflectivity RNP must be in 0.15 to 0.2 range. FIG. 4 shows the phase error introduced by the non-linear phase shifter as a function of the front-surface reflectivity. The illustrated by the graph, a reflectivity RNP of 0.2 produces the maximum allowable phase error of 5 degrees.
In interleavers constructed with an etalon cavity in the mirror arm, as illustrated in FIG. 2, an additional phase error is introduced by the etalon as a result of multiple-reflection effects, in addition to the intrinsic phase error φNP. This phase error at the extreme (because its extreme values affect channel isolation) is given by the relation
                                          ϕ            etalon                    =                      2            ⁢                                                  ⁢                                          tan                                  -                  1                                            ⁡                              [                                                      1                    2                                    ⁢                                      (                                                                                            (                                                                                    1                              +                                                              R                                                                                                                    1                              -                                                              R                                                                                                              )                                                                    -                                                                        (                                                                                    1                              -                                                              R                                                                                                                    1                              +                                                              R                                                                                                              )                                                                                      )                                                  ]                                                    ,                            (        3        )            where R is the reflectivity of the etalon's front surface 26. It should be noted that both cavities in the interleaver of FIG. 2 experience the effect of multiple reflections. Each phase error is measured with respect to the same mirror M2. Due to the location of this mirror, and the corresponding difference in optical paths, the phase-error equations are correspondingly different for each cavity. FIG. 5 is a plot of this phase error as a function of front-surface reflectivity. It is noted that a reflectivity of 0.25% produces a phase error of about 6 degrees, which degrades channel-isolation performance to an unacceptable degree.
Thus, the total phase error of a conventional interleaver (FIG. 2) within its stop band is given by the sum of the respective contributions from both arms of the device, as followsφerror=|φNP|+|φetalon|.  (4)FIG. 6 illustrates the dependence of channel isolation on the reflectivity of the etalon's front surface 26 when the GTR has a front surface 12 with a reflectivity RNP of 20% The graph shows that channel isolation is degraded by about 7 dB by a change in AR coating from zero to about 0.25% reflectivity. At 0.25% reflectivity, the isolation is only −20.5 dB, which is well below the typical requirement of 27 db in today's telecommunication system. In order to achieve the desired performance from a conventional interleaver, the etalon's front surface 26 has to exhibit a reflectivity better than 0.01% (−25.2 dB). This is almost impossible to implement with existing AR coatings.
To the extent that the resonator cavity Mc (see FIG. 1) is necessary to the function of an interferometric interleaver, the corresponding phase error is unavoidable. On the other hand, to the extent that a cavity structure is used for practical reasons only to precisely place the mirror M2, its contribution of ghost reflections from the front surface 26 of the etalon diminishes the usefulness of interferometric interleavers for channel isolation. Unfortunately, antireflective (AR) coatings are not sufficiently effective to control the reflectivity of a surface to the degree necessary to precisely control phase error. Therefore, there is still a need for an interferometric interleaver that minimizes the introduction of phase error as a result of its configuration, rather than through the use of AR coatings. The present invention provides simple solutions to that end.